Respiratory syncytial virus (RSV) infection is the primary cause of respiratory infection in young children, causing >120,000 hospitalizations in the US annually. RSV vaccine development has been hampered by the history of a failed vaccine trial in the early 1960s in which vaccinated children were not protected against subsequent natural infection but rather experienced more severe disease. Subsequent experiments in animal models and human subjects suggest the occurrence of severe RSV disease correlates with the induction of virus-specific Th2 CD4 T cells and eosinophil recruitment and degranulation. The G glycoprotein of RSV and killed virus vaccines containing adjuvants such as alum induce these types of immune responses associated with the vaccine-enhanced disease syndrome. Dendritic cells are important for processing antigen and initiating immune responses. Understanding their interaction with RSV may reveal the reasons for relatively short-lived immunity induced by natural RSV infection. CD4 T cells can influence and regulate the function of other immune effectors including CD8 T cells. In addition to the potential role of Th2 CD4 T cells in RSV pathogenesis as mentioned above, T regulatory cells have properties that suggest they may be important in RSV immunity and pathogenesis.